1. Field of the Invention
This invention relates to optical fiber distortion measurement devices and methods thereof, which measure optical fiber distortions in response to Brillouin backscattering light that occur when optical pulses are incident on measured optical fibers. This application is based on patent application No. Hei 9-172504 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
Recently, there are provided optical fiber distortion measurement devices that measure optical fiber distortions in response to Brillouin backscattering light. Herein, optical pulses are incident on optical fibers, in which Brillouin backscattering light occurs, which is detected and analyzed to measure the optical fiber distortions. This technique is established using the known property of the Brillouin backscattering light. That is, when a distortion occurs at a certain position of the optical fiber, frequency distribution (i.e., spectrum) of the Brillouin backscattering light that occurs at an arbitrarily selected position is shifted by a value corresponding to a distortion value as compared with the case where the optical fiber has no distortion.
FIG. 9 is a block diagram showing an example of a configuration of the optical fiber distortion measurement device. In FIG. 9, reference symbol "10" designates a light source that generates laser beams of continuous light, while "12" designates an optical coupler. The optical coupler 12 has two incoming terminals and two outgoing terminals. Herein, laser beams generated by the light source 10 are incident on a first incoming terminal of the optical coupler 12. The optical coupler 12 branches off the incoming laser beams to the outgoing terminals.
A first outgoing terminal of the optical coupler 12 is connected to an optical fiber 13. A second outgoing terminal of the optical coupler 12 is connected to a light frequency conversion circuit 14 by means of an optical fiber. The light frequency conversion circuit 14 converts light frequency of the incoming continuous light to produce a string of optical pulses, which have a prescribed light frequency difference. An outgoing terminal of the light frequency conversion circuit 14 is connected to an incoming terminal of an optical pulse extraction circuit 16 by an optical fiber. So, the optical pulse extraction circuit 16 extracts optical pulses having a prescribed light frequency v from the incoming optical pulse string thereof so as to output them.
An outgoing terminal of the optical pulse extraction circuit 16 is connected to an incoming terminal of an optical coupler 18. The optical coupler 18 has one incoming terminal and two incoming/outgoing terminals. Herein, a first incoming/outgoing terminal is connected to an optical fiber, which is optically connected to a measured optical fiber 20 by means of an optical connector 19. A second incoming/outgoing terminal of the optical coupler 18 is connected to an optical fiber 22.
The aforementioned optical fibers 13 and 22 are connected to a light receiving circuit 24. The light receiving circuit 24 has two photodiodes. Beams from the optical fiber 13 are incident on a first photodiode, while beams from the optical fiber 22 are incident on a second photodiode. The light receiving circuit 24 performs heterodyne wave detection on the incoming beams thereof and converts them to electric signals.
An amplifier circuit 26 amplifies the electric signals output from the light receiving circuit 24. An analog-to-digital conversion circuit (hereinafter, simply referred to as an A/D conversion circuit) performs sampling and quantization on the electric signals output from the amplifier circuit 26 so as to convert them to digital signals.
A signal processing circuit 30 performs signal processing such as noise elimination and logarithmic conversion on the digital signals output from the A/D conversion circuit 28. A curve approximation block 32 approximates light frequency characteristics of the measured optical fiber 20 in a form of a quadratic curve.
A peak frequency detection block 34 is provided to obtain a light frequency at a certain point of the approximated quadratic curve that has a peak value. A distortion value calculation block 36 calculates distortion values from measurement results. A display unit 38 is configured by the CRT (i.e., Cathode Ray Tube), or a liquid crystal display.
Next, a description will be given with respect to operation of the optical fiber distortion measurement device of FIG. 9.
Measurement of time-related variation waveform!
In FIG. 9, the continuous light output from the light source 10 branches off by the optical coupler 12, so that branch beams are produced. One of the branch beams is incident on the light receiving circuit 24, while another one is incident on the light frequency conversion circuit 14.
The light frequency conversion circuit 14 produces a string of optical pulses, which have a certain light frequency difference against the continuous light generated by the light source 1. So, an optical pulse string from the light frequency conversion circuit 14 is incident on the optical pulse extraction circuit 16, which extracts optical pulses having a prescribed light frequency .upsilon.. Thus, the optical pulse extraction circuit 16 outputs the optical pulses having the prescribed light frequency .upsilon., which are then incident on the measured optical fiber 20 by means of the optical coupler 18.
When the aforementioned optical pulses are incident on the measured optical fiber 20, Brillouin backscattering beams (or Brillouin backscattering light) occur at certain positions of the measured optical fiber 20 respectively. So, the Brillouin backscattering beams are sequentially incident on the light receiving circuit 24 by means of the optical coupler 18 while being delayed by times that are proportional to distances measured from the incoming terminal of the measured optical fiber 20 to the respective positions of the measured optical fiber 20.
Using the continuous light generated by the light source 1, the light receiving circuit 24 performs heterodyne wave detection sequentially on the Brillouin backscattering beams, which occur at the positions of the measured optical fiber 20 respectively. Thus, the light receiving circuit 24 produces electric signals, which are proportional to light intensities of the Brillouin backscattering beams that occur at the positions of the measured optical fiber 20 respectively. The amplifier circuit 28 amplifies the electric signals output from the light receiving circuit 24. Then, the A/D conversion circuit 28 performs analog-to-digital conversion on the electric signals amplified by the amplifier circuit 26 so as to convert them to digital signals.
The signal processing block 30 performs signal processing such as noise elimination and logarithmic conversion on electric signal values corresponding to the converted digital signals. Then, the signal processing block 30 performs plotting on the electric signal values in response to an elapsed time, which is measured from the incidence of the optical pulses, in other words, which corresponds to a distance measured from the incoming terminal of the measured optical fiber 20.
By the aforementioned processes, it is possible to obtain a time-related variation waveform of the Brillouin backscattering light, which occurs when the optical pulses having the light frequency v are incident on the measured optical fiber 20.
FIG. 10A and FIG. 10B show examples of the time-related variation waveform, which are visually displayed on a screen of the display unit 38. Herein, a horizontal axis represents a lapse of time that elapses from the incidence of the optical pulse. Elapsed times plotted on the time axis correspond to distances, which are measured from the incoming terminal of the measured optical fiber 20 to the positions of the measured optical fiber 20 respectively. A vertical axis represents light intensity of the Brillouin backscattering light, which occurs at each position of the measured optical fiber 20.
As shown in FIG. 10A, when the optical pulse is incident on the measured optical fiber 20, the light intensity of the Brillouin backscattering light that occurs in the measured optical fiber 20 becomes weak in response to the elapsed time, which is measured from the incidence of the optical pulse on the measured optical fiber 20. In other words, the light intensity of the Brillouin backscattering light becomes weak in response to the distance, which is measured from the terminal portion of the optical fiber 20.
Calculations of distortion value!
Next, a description will be given with respect to a calculation method to calculate a distortion value of the measured optical fiber 20.
When calculating the distortion value of the measured optical fiber 20, the optical fiber distortion measurement device of FIG. 9 uses the light frequency conversion circuit 14 to repeat the aforementioned operations while changing the light frequency v of the optical pulses incident on the measured optical fiber 20 sequentially by a prescribed value.
Thus, the aforementioned time-related variation waveform can be obtained with respect to each of multiple light frequencies.
FIG. 11 is a three-dimensional graph showing an example of time-related variation waveforms that are formed in response to the multiple light frequencies respectively. Herein, a horizontal axis represents the light frequency .upsilon. of the optical pulse incident on the measured optical fiber 20, while a vertical axis represents light intensity of the Brillouin backscattering light. Another axis "X1" that crosses the above horizontal and vertical axes represents a lapse of time measured from the incidence of the optical pulse(s), in other words, a distance measured from the incoming terminal of the measured optical fiber 20 (i.e., a position within the measured optical fiber 20). That is, a coordinates plane formed between the vertical axis and the axis X1 of FIG. 11 corresponds to a coordinates plane shown in FIG. 10A and FIG. 10B.
In FIG. 11, a point D corresponding to a prescribed distance measured from the incoming terminal of the measured optical fiber is plotted on the axis X1. FIG. 12 is a graph that is formed by cutting the three-dimensional graph of FIG. 11 in a round slice in accordance with a plane perpendicular to the axis X1 at the point D. That is, FIG. 12 shows a waveform (i.e., spectrum waveform) representing frequency distribution (i.e., spectrum) of Brillouin backscattering light with respect to the point D.
Through the aforementioned processes, the spectrum waveform (see FIG. 12) is obtained with respect to the prescribed distance D. The curve approximation block 32 shown in FIG. 9 puts data of the spectrum waveform into a quadratic formula to produce an approximated curve (i.e., quadratically approximated curve) of the spectrum waveform. Then, the peak frequency detection block 34 differentiates the approximated curve to produce a light frequency (denoted by "peak frequency .upsilon..sub.p ") at which light intensity of the Brillouin backscattering light indicates a maximum value.
Lastly, the distortion calculation block 36 puts the peak frequency .upsilon..sub.p, which is detected by the peak frequency detection block 34, into an equation
(1) so as to calculate a distortion value .epsilon., as follows: ##EQU1## where .upsilon..sub.b represents a peak frequency (which is a fixed value for the measured optical fiber 20) when the measured optical fiber 20 has no distortion, while K represents a distortion coefficient.
Through the above process, it is possible to calculate the distortion value .epsilon. with respect to the prescribed distance in the measured optical fiber 20 (i.e., distance D measured from the incoming terminal of the measured optical fiber 20), so that the distortion value is displayed on the screen of the display unit 38.
The foregoing is the description of the operation of the optical fiber distortion measurement device of FIG. 9.
As described above, the aforementioned optical fiber distortion measurement device calculates the distortion value of the measured optical fiber. In that case, every time the measurement is performed, it is necessary to measure time-related variation waveforms of the aforementioned Brillouin backscattering beams with respect to multiple optical frequencies while sequentially changing the light frequency .upsilon.. Sometimes, it is necessary to measure the time-related variation waveforms with respect to forty to one hundred kinds of light frequencies.
About two to three seconds is required to measure one time-related variation waveform with respect to one light frequency. In order to measure the distortion value of the measured optical fiber, it is necessary to provide forty-times to one-hundred-times longer time, e.g., maximally six minutes.
As described heretofore, the aforementioned optical fiber distortion measurement device requires much time to measure the distortion value of the measured optical fiber. So, the device suffers from poor efficiency.
Particularly, when the device is used as a fiber sensor, it is important for the measurement time to be short. Thus, there is a problem that the aforementioned optical fiber distortion measurement device cannot be employed as the fiber sensor.